High density stainless steel product and method for the preparation thereof

ABSTRACT

The invention concerns a method of preparing compacts having a sintered density of above 7.3 g/cm 3.  This method comprises the steps of subjecting an annealed, water-atomised, essentially carbon free stainless steel powder, which in addition to iron, comprises at least 10% by weight of chromium, not more than 0.4%, preferably not more than 0.3% by weight of oxygen, not more than 0.05%, preferably not more than 0.02% and most preferably not more than 0.015% of carbon, not more than 0.5% by weight of Si and not more than 0.5% of impurities, to HVC compaction with an uniaxial pressure movement with a ram speed of at least 2 m/s, and sintering the green body.

FIELD OF THE INVENTION

[0001] This invention relates to the general field of powder metallurgy.Particularly the invention is concerned with high-density stainlesssteel products and a compacting and sintering operation for achievingsuch products.

BACKGROUND OF THE INVENTION

[0002] Currently used methods for preparing high density products, suchas flanges, of stainless steel powders involve compacting the stainlesssteel powders to densities of between about 6.4 and 6.8 g/cm³ atcompaction pressures of 600-800 MPa. The obtained green body is thensintered at high temperatures, i.e. temperatures between about 1250° C.and 1400° C. for 30 to 120 minutes in order to get densities of about7.25 g/cm³. The requirement for the long sintering times at thecomparatively high temperatures is of course a problem considering thehigh energy costs. The necessity for special, high temperature furnacesis another problem.

OBJECTS OF THE INVENTION

[0003] An object of the invention is to provide a solution to theseproblems and provide a method for the preparation of high-densityproducts, particularly products having a sintered density above 7.25,preferably above 7.30 and most preferably above 7.35 g/cm³.

[0004] A second object is to provide a compaction method adapted toindustrial use for mass production of such high-density products.

[0005] A third object is to provide a process for the sintering of suchcompacted products requiring less energy.

[0006] A fourth object is to provide a process for sintering thestainless steel compacts to densities above about 7.25 g/cm³ which canbe performed in conventional furnaces without need for special hightemperature equipment.

[0007] A fifth object is to provide a process for the manufacturing oflarge sintered stainless steel PM products, such as flanges, having arelatively simple geometry.

SUMMARY OF THE INVENTION

[0008] In brief the method of preparing such high density productscomprises the steps of

[0009] subjecting an annealed, water-atomised stainless steel to HVCcompaction with a uniaxial pressure movement at an impact ram speedabove 2 m/s;

[0010] and sintering the green body.

DETAILED DESCRIPTION OF THE INVENTION

[0011] The annealed stainless steel powder is preferably an essentiallycarbon free stainless steel powder which in addition to iron comprisesat least 10% by weight of chromium, not more than 0.4%, preferably notmore than 0.3% by weight of oxygen, not more than 0.05%, preferably notmore than 0.02% and most preferably not more than 0.015% of carbon, atmost 0.5% by weight of Si and not more than 0.5% of impurities. Suchpowders and the preparation thereof are described in the internationalpatent publication WO 98/58093 that is hereby incorporated by reference.

[0012] Specifically the annealed powder could include, by percent ofweight, 10-30% of chromium, 0-5% of molybdenum, 0-15% of nickel, 0-0.5%of silicon, 0-1.5% of manganese, 0-2% of niobium, 0-2% of titanium, 0-2%of vanadium and at most 0.3% of inevitable impurities and mostpreferably 10-20% of chromium, 0-3% of molybdenum, 0.1-0.3% of silicon,0.1-0.4% of manganese, 0-2% of niobium, 0-0.5% of titanium, 0-0.5% ofvanadium, 0-8% of tungsten and essentially no nickel or alternatively7-10% of nickel.

[0013] A preferred annealed powder which may be used comprises, bypercent of weight, 10-20% of chromium, 0-3% of molybdenum, 0.1-0.3% ofsilicon, 0.1-0.4% of manganese, 0-2% of niobium, 0-0.5% of titanium,0-0.5% of vanadium, 0-8% of tungsten and essentially no nickel or,alternatively, 7-10% by weight of nickel, the balance being iron.

[0014] In order to obtain the products having the desired high densityaccording to the present invention the compacting method is important.Normally used compaction equipment does not work quite satisfactorily,as the strain on the equipment will be too great. It has now been foundthat the high densities required may be obtained by the use of thecomputer controlled percussion machine disclosed in the U.S. Pat. No.6,202,757 which is which is hereby incorporated by reference.Particularly, the impact ram of such a percussion machine may be usedfor impacting the upper punch of a die including the powder in a cavityhaving a shape corresponding to the desired shape of the final compactedcomponent. When supplemented with a system for holding a die, e.g. aconventionally used die, and a unit for powder filling (which may alsobe of conventional type) this percussion machine permits an industriallyuseful method for production of high-density compacts. An especiallyimportant advantage is that, in contrast to previously proposed methods,this arrangement driven by hydraulics permits mass production(continuous production) of such high density components.

[0015] In the U.S. Pat. No. 6,202,757 it is stated that the use of thepercussion machine involves “adiabatic” moulding. As it is not fullyclarified if the compaction is adiabatic in a strictly scientificmeaning and we have used the term high velocity compaction (HVC) forthis type of compaction wherein the density of the compacted product iscontrolled by the impact energy transferred to the powder.

[0016] According to the present invention the ram speed should be above2 m/s. The ram speed is a manner of providing energy to the powderthrough the punch of the die. No straight equivalence exists betweencompaction pressure in a conventional press and the ram speed. Thecompaction which is obtained with this computer controlled HVC depends,in addition to the impact ram speed, i.a. on the amount of powder to becompacted, the weight of the impact body, the number of impacts orstrokes, the impact length and the final geometry of the component.Furthermore, large amounts of powder require more impacts than smallamounts of powder. Thus the optimal conditions for the HVC compactioni.e. the amount of kinetic energy which should be transferred to thepowder, may be decided by experiments performed by the man skilled inthe art. Contrary to the teaching in the U.S. Pat. No. 6,202,757 thereis, however, no need to use a specific impact sequence involving a lightstroke, a high energy stroke and a medium-high energy stroke for thecompaction of the powder. According to the present invention the strokes(if more than one stroke is needed) may be essential identical andprovide the same energy to the powder.

[0017] Experiments with existing equipment has permitted ram speeds upto 30 m/s and, as is illustrated by the examples, high green densitiesare obtained with ram speeds about 10 m/s. The method according to theinvention is however not restricted to these ram speeds but it isbelieved that ram speeds up to 100 or even up to 200 or 250 m/s may beused. Ram speeds below about 2 m/s does, however, not give thepronounced effect of densification. It is preferred that the ram speedabove 3 m/s. Most preferably the ram speed is above 5 m/s.

[0018] The compaction may be performed with a lubricated die. It is alsopossible to include a suitable lubricant in the powder to be compacted.Alternatively, a combination thereof may be used. Alternatively theparticles may be provided with a coating. This coating or film isachieved by mixing the powder composition, which includes the free orloose, non agglomerated powder particles with the lubricant, subjectingthe mixture to an elevated temperature for melting the lubricant andsubsequently cooling the obtained mixture during mixing for solidifyingthe lubricant and providing the powder particles or aggregates thereofwith a lubricant film or coating. The lubricant can be selected amongconventionally used lubricants such as metal soaps, waxes andthermoplastic materials, such as polyamides, polyimides, polyolefins,polyesters, polyalkoxides, polyalcohols. Specific examples of lubricantsare zinc stearate, H-wax® and Kenolube®. The amount of lubricant mayvary up to 1% by weight of the powder composition.

[0019] Furthermore the compaction may be performed at ambient or atelevated temperature e.g. between 90 and 180° C. In the latter case apre-heated powder composition is subjected to compaction in a pre-heateddie and the lubricant may be selected from lubricants specificallydeveloped to this end. Examples of such warm compaction lubricants aredisclosed in e.g. the U.S. Pat. No. 5,154,881 and 5,744,433.

[0020] Generally the subsequent sintering may be performed at atemperature between about 1120 and 1250° C. for a period between about30 and 120 minutes. According to a preferred embodiment the sintering isperformed in a belt furnace at temperatures below 1180° C., preferablybelow 1150° C. and most preferably below 1160° C. The invention ishowever not restricted to sintering at such low temperatures and bysintering at higher temperatures, such as up to 1400° C. even higherdensities may be obtained. It is also preferred that the sinteringatmosphere includes hydrogen. Preferably a hydrogen/nitrogen atmosphereshould be used.

[0021] A particular advantage of the invention is that the compactshaving near theoretical density may be sintered at low temperatures,such as 1120-1150° C., in conventional furnaces, such as belt furnaces.This is in contrast to conventional compaction methods where it is notpossible to obtain such high green densities and where a high sintereddensity is obtained by high temperature sintering, which causesshrinkage of the compacts. By using the HVC compaction method with no ora very small amount of lubricant included in the powder composition tobe compacted, the green density will be essentially identical with thesintered density. This in turn means that very good tolerances areobtained.

[0022] The method according to the invention permits the manufacture ofgreen and sintered compacts having high density, such as densities above96 or even above 98% of the theoretical density. For stainless steelpowders having the usual chemical compositions this corresponds todensities above 7.25, 7.30 and even 7.35 g/cm³

[0023] The invention as described in the present specification and theappended claims is believed to be of especial importance for largesintered stainless steel PM compacts having a comparatively simplegeometry, such as flanges. Other products which may be of interest aregas-tight oxygen probes. The invention is, however, not limited to suchproducts.

[0024] The invention is further illustrated by the following example:

EXAMPLE 1

[0025] Two powders, A and B respectively, having the compositions givenin the following table were subjected to HVC compaction using acompaction machine Model HYP 35-4 from Hydropulsor AB, Sweden. TABLE 1Material C Ni Fe Mn Cr Si O N A 0.01 0.1 Base 0.1 11.4 0.9 0.19 0.036 B0.005 0.05 Base 0.1 11.2 0.13 0.20 0.001

[0026] A is a standard stainless powder

[0027] B is an annealed stainless powder which should be used accordingto the invention

[0028] The die was lubricated with zinc stearate dissolved in acetone.After drying 70 g of the powder was poured into the die. As can be seenfrom the following tables 2-5 the method of according to the presentinvention makes it possible to obtain stainless steel products havingboth green and sintered densities above 7.3 g/cm³. These tables alsoshows the impact of the stroke length on the density. The strokelengths, which varied between 10 and 70 mm, correspond to ram speedsbetween about 3 and about 8 m/s. TABLE 2 Powder Theor. density 7.75g/cm³ A Stroke Green Density Sample length (mm) (g/cm³) Rel dens (%) 110 5.50 71.0 2 20 6.06 78.1 3 30 6.41 82.7 4 40 6.67 86.0 5 50 6.91 89.26 60 7.12 91.8 7 65 7.15 92.2 8 70 7.21 93.0

[0029] TABLE 3 Powder Theor. density 7.78 g/cm³ B Stroke Green DensitySample length (mm) (g/cm³) Rel dens (%) 1 10 5.86 75.3 2 20 6.44 82.8 330 6.81 87.6 4 40 7.10 91.3 5 50 7.27 93.4 6 55 7.35 94.5 7 60 7.41 95.38 65 7.41 95.2

[0030] Sintered properties of Powder A are disclosed in the followingtable 3. TABLE 3 Temp Sintered density Rel. Dens. Sample (° C.) time(min) (g/cm³) (%) 1 1250 30 5.56 71.7 2 1150 30 6.04 77.9 3 1250 30 6.5284.2 4 1150 30 6.66 86.0 5 1250 30 7.02 90.6 6 1150 30 7.10 91.6 7 125030 7.22 93.2 8 1150 30 7.19 92.8

[0031] Sintered properties of Powder B are disclosed in the followingtable 4. TABLE 4 Temp Sintered density Rel. Dens. Sample (° C.) time(min) (g/cm³) (%) 1 1250 30 5.93 76.3 2 1150 30 6.42 82.5 3 1250 30 6.8788.3 4 1150 30 7.06 90.7 5 1250 30 7.33 94.2 6 1150 30 7.32 94.1 7 125030 7.45 95.8 8 1150 30 7.39 95.0

1. Method of preparing compacts having a high density comprising thesteps of subjecting an annealed, water-atomised, essentially carbon freestainless steel powder, which in addition to iron, comprises at least10% by weight of chromium, not more than 0.4%, preferably not more than0.3% by weight of oxygen, not more than 0.05%, preferably not more than0.02% and most preferably not more than 0.015% of carbon, at most 0.5%by weight of Si and not more than 0.5% of impurities to HVC compactionwith a uniaxial pressure movement with an impact ram speed above 2 m/s;and sintering the green body.
 2. Method according to claim 1 wherein theannealed powder comprises, by percent of weight 10-30% of chromium 0-5%of molybdenum 0-15% of nickel 0-0.5% of silicon 0-1.5% of manganese 0-2%of niobium 0-2% of titanium 0-2% of vanadium 0-8% of tungsten and atmost 0.3% of inevitable impurities, the balance being iron.
 3. Methodaccording to any one of the claims 1-2 wherein the annealed powdercomprises, by percent of weight 10-20% of chromium 0-3% of molybdenum0.1-0.45% of silicon 0.1-0.4% of manganese 0-2% of niobium 0-0.5% oftitanium 0-0.5% of vanadium 0-8% of tungsten and essentially no nickelthe balance being iron.
 4. Method according to any one of the claims 1-2wherein the annealed powder comprises comprising, by percent of weight,10-20% of chromium 0-3% of molybdenum 0.1-0.3% of silicon 0.1-0.4% ofmanganese 0-2% of niobium 0-0.5% of titanium 0-0.5% of vanadium 0-8% oftungsten and 7-10% of nickel the balance being iron.
 5. Method accordingto any one of the claims 1-4 characterised in that the compaction isperformed at a ram speed above 3, preferably above 5 m/s.
 6. Methodaccording to any one of the claims 1-5 characterised in that thecompaction is performed as warm compaction.
 7. Method according to anyone of the preceding claims for the preparation of compacts having adensity above about 96%, preferably above 98% of the theoreticaldensity.
 8. Method according to any one of the claims 1 to 7 wherein thesintering is performed at a temperature between about 1120 and 1250° C.for a period between about 30 and 120 minutes.
 9. Method according toany one of the claims 1 to 8 characterised in that the sintering isperformed in a belt furnace at temperatures below 1250° C., preferably,below 1200° C. and most preferably below 1160° C.
 10. Method accordingto any one of the claims 1 to 10 wherein the sintering atmosphereincludes hydrogen.
 11. Compacted and sintered stainless steel products,such as flanges, having a sintered density of above 7.25 g/cm³,preferably above 7.30 g/cm³ and comprising at least 10% by weight ofchromium, not more than 0.05%, preferably not more than 0.02% and mostpreferably not more than 0.015% of carbon, and not more than 0.5% byweight of silicon.
 12. Compacted and sintered bodies, according to claim11 having a composition, by percent of weight, of 10-30% of chromium0-5% of molybdenum 0-15% of nickel 0-0.5% of silicon 0-1.5% of manganese0-2% of niobium 0-2% of titanium 0-2% of vanadium 0-8% of tungsten thebalance being iron and at most 0.3% of inevitable impurities 13.Compacted and sintered bodies according to claim 12 including 10-20% ofchromium 0-3% of molybdenum 0.1-0.3% of silicon 0.1-0.4% of manganese0-2% of niobium 0-0.5% of titanium 0-0.5% of vanadium 0-8% of tungstenand essentially no nickel, the balance being iron and at most 0.3% ofinevitable impurities.
 14. Compacted and sintered bodies according toclaim 12 including 10-20% of chromium 0-3% of molybdenum 0.1-0.3% ofsilicon 0.1-0.4% of manganese 0-2% of niobium 0-0.5% of titanium 0-0.5%of vanadium 0-8% of tungsten and 7-10% of nickel, the balance being ironand at most 0.3% of inevitable impurities